Bluebird Battles : Aggression pays on the bluebird frontier, but not in family life

To human observers, bluebirds might seem like the least pugnacious of birds, but in reality they lead strife-ridden lives. This is especially true during the breeding season, when they must defend their nest sites from rivals attempting to evict them, such as other bluebirds, Tree Swallows or House Sparrows.

Male bluebirds respond in different ways to such threats. In anthropomorphic terms, some are bullies and some are wimps. For example, if researchers place a model of a Tree Swallow on a series of bluebird nest boxes, some male bluebirds will attack the model, whereas others are less aggressive.

A new study by Montana-based researchers Renee Duckworth and Alexander Badyaev shows how these "personality" differences have allowed Western Bluebirds to expand their range at the expense of Mountain Bluebirds

Western Bluebirds (left) are generally dominant over
Mountain Bluebirds (right) and outcompete them for nest sites
in Montana's valleys.

Both species originally occurred in western Montana, but over much of the 20th century they became rare in the valleys as the number of appropriate nest sites decreased. During that period, Mountain Bluebirds persisted at higher elevations where nest cavities were more common, while Western Bluebirds essentially vanished from the state.

Over the past several decades, however, the valleys of Montana have become prime bluebird habitat as forestry practices have become more bluebird-friendly and more people have set up artificial nest boxes for the birds. Mountain Bluebirds were the first to take advantage of these new opportunities by expanding back into the valleys from the adjacent mountain slopes. They have since been joined by Western Bluebirds, which have steadily moved back into the region from the south and west.

Although the Western Bluebirds are the more recent arrivals, Duckworth and Badyaev showed that they are generally dominant over the Mountain Bluebirds, and over time tend to exclude them from prime valley real estate.

How does this all relate to the fact that some Western Bluebirds are more aggressive than others? By studying pedigrees of bluebirds monitored over many generations, the researchers have discovered that bluebird aggressiveness is largely inherited: aggressive males have aggressive sons. The more aggressive birds are also the better dispersers. This means that the most aggressive birds have led the way during the species' range expansion back into Montana, and into territorial conflict with Mountain Bluebirds living in the valleys. For Western Bluebirds existing on the frontier, aggressiveness has a distinct advantage.

But in a fascinating twist, the story changes entirely when the bluebird frontier becomes well settled. The aggressiveness that confers a competitive advantage at the edge of the range expansion is actually a detriment in areas occupied by a stable Western Bluebird population. Once Western Bluebirds become well established, offspring tend to settle near their close relatives, and some even engage in cooperative breeding. In a high-density Western Bluebird population, the more aggressive males continue to battle and therefore spend less time caring for their nestlings. In this context, they are poor fathers, and therefore have fewer surviving offspring than their less aggressive neighbors.

This difference is readily apparent across the regions of Montana that Duckworth and Badyaev study. Natural selection favors aggression at the leading edge of the species' expanding range, but it favors lower aggression once populations become well established, and neighbors are friends and family.

SOURCE : Cornell Lab of Ornithology